A material's properties can be summarized by a material parameter known as the “wave impedance” (or simply “impedance”) of the material, which is a complex number. A good discussion of the relationship between wave impedance and permittivity, permeability, and conductivity is available, at the time of writing this disclosure, in the Wikipedia entry for wave impedance: http//en.wikipedia.org/wiki/wave impedance.
Many materials of interest are nonconductive and nonmagnetic. For such materials, permeability does not play a role, and there is a simple, one-to-one relationship between the impedance of the material and the material's permittivity, such that measuring the permittivity and measuring the impedance of the material are equivalent. Impedance spectroscopy is also often referred to as dielectric spectroscopy and vice versa.
Instruments for determining the wave impedance of a material have been developed using many different sensing technologies that comprise separate components operating with different electromagnetic modalities. These instruments include capacitive sensors wherein the resonant tank circuit of a Colpitts, Clapp, or Hartley LC oscillator is affected by exposure to a material of interest. The wave impedance of the material affects the oscillation frequency the LC oscillator. An example of such a sensor is disclosed by in U.S. Pat. No. 5,418,466 wherein a tuned circuit oscillates at a frequency representative of the complex dielectric constant of the medium.
In applications wherein the objective is to sense both the real and imaginary parts of a wave impedance, multiple measurements are required. It is fundamentally impossible to derive the values of two independent unknown quantities from a single measurement. In general, sensors such as a capacitance sensor wherein a material of complex permittivity affects the oscillation frequency of an LC resonant circuit, accuracy for measurement of complex impedance is limited due to nonlinear cross modulation effects relating to the real and imaginary parts of material permittivity.
Another class of instruments for sensing wave impedance of a material is based on time delay reflectometry (TDR) and time delay transmissometry (TDT) wherein the propagation speed of a wave along a transmission line is measured. When the transmission line is exposed to a material, the propagation speed of signal through the transmission line is affected. TDR and TDT sensors generally require delay measurements in the picosecond range for useful accuracy. limitations for TDR and TDT sensing include requirement for a length of transmission line adequate to obtain sufficient propagation delay and economic considerations involved with maintaining stability within a picosecond delay timer.
An application wherein wave impedance affects a communication link is disclosed in U.S. Pat. No. 6,593,886. The apparatus disclosed here is a planar loop antenna with a balun.
A prior art example of a sensor antenna coupled into an adjacent medium is disclosed in U.S. Pat. No. 9,916,528. The signal strength of RF energy emitted by an RFID tag is affected by the frozen or thawed state of a material disposed proximally with the tag.
An RFID sensor arrangement for determining a degradation condition within a material is disclosed in U.S. Patent Application 2010/0090802 A1. Reference to a material wave impedance is not detailed.
An early paper disclosing a split ring resonator with negative permittivity is D. R. Smith et al, “Determination of negative permittivity and permeability of metamaterials from reflection and transmission coefficients”, Physical Review B, November, 2001; doi: 10.1103/PhysRevB.65.195104.
There is a need for improved impedance spectrometry for determining permittivity of a material with improved accuracy and lower cost for sensing the real and imaginary parts of the wave impedance of a material. Instruments are needed to provide increased accuracy, reduced cost, portability, imbedded sensing and operation within networks including a mesh drone network and a WLAN cellular network.